Monazite Structure Research Articles

Simulation of thermodynamic mixing properties of actinide-containing zircon solid solutions

Solid solutions among zircon (ZrSiO4, I41/amd) and zircon-structured orthosilicates ASiO4 (A = Hf, Th, U, Pu, or Ce) are important to a wide variety of applications: nuclear materials, geochronology, and even electronic materials. The thermodynamic mixing properties of the following seven binary solid solutions were simulated using density functional theory followed by Monte Carlo modeling and thermodynamic integration: (Zr,Hf)SiO4, (Zr,Th)SiO4, (Zr,U)SiO4, (Zr,Pu)SiO4, (Zr,Ce)SiO4, (Hf,Pu) SiO4, and (Th,U)SiO4. ZrSiO4 and HfSiO4 were found to form a nearly ideal solid solution, but the miscibility of the other solid solutions was limited to no more than 12 mol% of the substituting cation. The binaries were ranked by extent of miscibility: (Zr,Hf)SiO4 > (Th,U)SiO4 > (Zr,Pu)SiO4 > (Zr,Ce) SiO4 > (Hf,Pu)SiO4 > (Zr,U)SiO4 > (Zr,Th)SiO4. The extent of solid solution has been estimated for each binary. The end-members PuSiO4, CeSiO4, and USiO4 were determined to be unstable relative to a mixture of SiO2(quartz) and crystalline PuO2, CeO2(cerianite), or UO2(uraninite). Isostructural thorite (ThSiO4) is calculated to be marginally stable, but its monoclinic polymorph, huttonite, which has the structure of monazite, is marginally unstable relative to SiO2(quartz) and ThO2(thorianite) at 0 K. If thermodynamic equilibrium could be reached at low temperatures, exsolution textures would appear in most zircon solid solutions perpendicular to [001], but perpendicular to a linear combination of [100] and [010] in (Hf,Pu)SiO4.

METAMORPHIC-HYDROTHERMAL REE MINERALS IN THE BACUCH MAGNETITE DEPOSIT, WESTERN CARPATHIANS, SLOVAKIA: (Sr,S)-RICH MONAZITE-(Ce) AND Nd-DOMINANT HINGGANITE

Accessory REE minerals occur in a small metamorphic magnetite ore deposit at Bacuch, Veporic Superunit, central Slovakia. We distinguish two populations of monazite. Monazite I forms subhedral to euhedral crystals associated with magnetite. It contains ≤12 wt.% ThO2, ≤2.7% UO2, ≤0.85% SO3, with low Ca and Sr contents. Compared to the common monazite-(Ce) I, monazite-(Nd) I (≤26.1% Nd2O3) occasionally occurs with an atomic ratio Nd:Ce up to 1.17. Monazite II is present as irregular aggregates with hingganite in younger hydrothermal quartz – albite – chlorite veinlets, or as rim zones on monazite I. Monazite II is depleted in Th and U and has an unusually high content of S (≤11.3% SO3, 0.31 apfu S) and Sr (≤8.7% SrO, 0.18 apfu Sr). This composition indicates a (Ca,Sr)S(REE,Y)−1P−1 substitution as a dominant mechanism of Sr and S entry into the monazite structure. Some monazite II crystals display an elevated Eu content (≤1.2% Eu2O3). Xenotime-(Y) forms subhedral crystals, in association with monazite-(Ce) I, magnetite, pyrite transformed to goethite (?) and quartz. Gadolinite-group minerals at Bacuch are represented by hingganite with an atomic value of X □/ X (□ + Fe) in the range 0.51–0.72. Neodymium is locally the most abundant REE (17.8–18.7% Nd, ~0.56 apfu ), and an Nd-dominant member of the gadolinite group was identified. The composition of hingganite-(Y) was also determined. The principal mechanism of substitution in hingganite is Fe2+O2□−1(OH)−2. Primary monazite I and xenotime are most likely products of regional metamorphism, together with magnetite mineralization. On the contrary, Sr- and S-rich monazite II and hingganite originated during a younger (Alpine) metamorphic-hydrothermal overprint in a fluid-rich regime.

Influence of Calcining Temperature on the Microstructure of LaPO<sub>4</sub> Powders

Nanometer LaPO4 powders were synthesized by liquid-liquid direct precipitation method. La(NO3)3•6H2O and (NH4)3PO4•3H2O were used as raw materials. The calcining temperature was 900°C, 1000°C, 1100°C, respectively. DTA result shows that the LaPO4 precursor is LaPO4•4H2O. The calicined powders were characterized by X-ray powder diffraction (XRD) and Scanning Electron Microscope (SEM), and exhibited a pure LaPO4 phase with a monazite structure about 50-100 nm in diameter size. With the calcining temperature increasing, the crystallization of the LaPO4 became better and the grain shape changed from elongated grain shape to spherical grain shape.

Bonding Characteristics, Thermal Expansibility, and Compressibility of RXO4 (R = Rare Earths, X = P, As) within Monazite and Zircon Structures

Systematically theoretical research was performed on the monazite- and zircon-structure RXO(4) (R = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; X = P, As) series by using the chemical bond theory of dielectric description. The chemical bond properties of R-O and X-O bonds were presented. In the zircon phase, the covalency fractions of X-O bonds increased in the order of V-O < As-O < P-O, which was in accordance with the ionic radii and electronegative trends, and the covalency fractions of R-O bonds varied slightly due to the lanthanide contraction. While in the monazite phase, both R-O and X-O bonds were divided into two groups by their covalency fractions. The contributions from the bond to the lattice energy, linear thermal expansion coefficient (LTEC), and bulk modulus were explored. The X-O bonds with short bond lengths and high chemical valence made greater contributions to the lattice energy and performed nearly rigidly during the deformation. A regular variation of lattice energy, LTEC, and bulk modulus with the ionic radii of the lanthanides was observed in both monazite and zircon phases.

X-ray diffraction and infrared spectroscopy of monazite-structured CaSO 4 at high pressures: Implications for shocked anhydrite

X-ray diffraction and infrared spectroscopy of CaSO 4 are conducted to pressures of 28 and 25 GPa, respectively. A reversible phase transition to the monoclinic monazite-structure occurs gradually between 2 and ∼5 GPa with a highly pressure-dependent volume change of ∼6–8%. A second-order fit of the X-ray data to the Birch–Murnaghan equation of state yields a bulk modulus ( K) of 151.2 (±21.4) GPa for the high-pressure monoclinic phase. In the high-pressure infrared spectrum, the infrared-active asymmetric stretching and bending vibrations of the sulfate tetrahedra split at the phase transition, in accord with the results of factor group analysis. Additionally, the tetrahedral symmetric stretching vibration, which is weak in the anhydrite phase, becomes strongly resolved at the transition to the monazite structure. The infrared results indicate that the sulfate tetrahedra are more distorted in the monazite-structured phase than in anhydrite. Kinetic calculations indicate that the anhydrite to monazite transformation may generate the phase transition observed near 30 GPa under shock loading in CaSO 4. Our results indicate that the anhydrite- and monazite-structured phases may be the only phases that occur under shock loading of CaSO 4 to pressures in excess of 100 GPa.

Synthesis of rare earth phosphates in molten LiCl–KCl eutectic: Application to preliminary treatment of chlorinated waste streams containing fission products

The precipitation of rare earth phosphates (RE = La, Ce, Pr, Nd, and Lu), from RECl 3 was investigated in molten LiCl–KCl eutectic at 500 °C in air. Ammonium dihydrogenphosphate (NH 4H 2PO 4) was used as the phosphorus precursor. X-ray diffraction analysis indicated the formation of compounds with monazite (La, Ce, Pr, and Nd) or xenotime (Lu) structures. 31P NMR spectroscopy measurements confirmed that lanthanum formed pure monazite, which indicates a LaCl 3 → LaPO 4 conversion factor near 100%. These results demonstrate that the stoichiometric addition of NH 4H 2PO 4 is sufficient to obtain quantitative precipitation at 500 °C of anhydrous rare earth phosphates in molten LiCl–KCl. The use of this type of precursor, which has the advantage of not modifying the chemical composition of the medium after recovery of the rare earth phosphates, could be considered during the first step of purification of chlorinated baths containing fission products arising from spent fuel reprocessing by a pyrochemical process.

Sintering, microstructure and microwave dielectric properties of rare earth orthophosphates, RePO 4 (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb)

Various rare earth orthophosphates, such as monazite and xenotime RePO 4 (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb) were prepared by a conventional solid-state reaction method. The crystal structure and microstructure of the sintered ceramics were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The dielectric properties were measured in the microwave region using a network analyzer. It was found that the monazite RePO 4 could be sintered near 1400 °C, although the xenotime RePO 4, which had a smaller Re 3+ ionic radius, could be sintered above 1600 °C. The permittivity ( ɛ r) of the monazite structures was higher than that of the xenotime structures. This difference was explained by the differences in ionic polarizability and bond strength. Both monazite and xenotime rare earth orthophosphates, however, exhibited a high quality factor ( Q × f), where Q = 1/tan δ and f is the measuring frequency, of greater than 60,000 GHz. The temperature coefficient of resonant frequency ( τ f ) was a negative value, ranging between −17 and −56 ppm/°C.

Experimental study of the dehydration reactions gypsum-bassanite and bassanite-anhydrite at high pressure: Indication of anomalous behavior of H2O at high pressure in the temperature range of 50–300°C

The system CaSO(4)-H(2)O, characterized by the three dehydration reactions gypsum-anhydrite, gypsum-bassanite, and bassanite-anhydrite, was reexamined by in situ differential pressure analysis in the temperature range of 60-350 degrees C up to 3.5 GPa pressure. The investigation revealed a fine structure in the dehydration boundaries of gypsum-bassanite and bassanite-anhydrite, each characterized by three inflections at 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In addition, the phase transition of anhydrite high pressure anhydrite (monazite structure) was established for the first time at high P-T conditions intersecting the bassanite-anhydrite dehydration boundary at 2.15 GPa250 degrees C. Furthermore, the triple point gypsum-bassanite-anhydrite was redetermined with 235 MPa80.5 degrees C. The evaluation of the gypsum-bassanite dehydration boundary with respect to the volume and entropy change of the reaction, DeltaV(react) and DeltaS(react), by means of the Clausius-Clapeyron relation yields for the entropy parameter an unusually large increase over the range of the noted inflections. This is interpreted as anomalous entropy behavior of H(2)O related presumably to a dramatic increase in fluctuations of the hydrogen network of the liquid leading possibly into a new structural state. The effect is strongly related to the three noted pressure levels of 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In a synopsis of data including also a previous high pressure study in the temperature range between 0 and 80 degrees C, a tentative P-T diagram of H(2)O is proposed.

Plutonium in monazite and brabantite: Diffuse reflectance spectroscopy study

The valence states of plutonium (Pu) in monazite and brabantite have been studied by using diffuse reflectance spectroscopy. In the absence of charge compensators, Pu exists mainly as trivalent ions in air-fired monazites and only trivalent ions in argon-fired monazites. However, with added Ca 2+/Pb 2+ as charge compensators Pu can exist as tetravalent ions. Published data on absorption spectra of both trivalent and tetravalent Pu ions in the monazite structure have been extended to the mid near infrared region (4000 cm −1).

Self-Irradiation of Ceramics and Single Crystals Doped With Pu-238: Summary of 5 Years of Research of the V. G. Khlopin Radium Institute

AbstractTo investigate the resistance of actinide host phases to accelerated radiation damage, which simulates radiation induced effects of long term storage, the following samples doped with plutonium-238 (from 2 to 10 wt. %) have been repeatedly studied using XRD and other methods: cubic zirconia, Zr0.79Gd0.14Pu0.07O1.99; monazite, (La,Pu)PO4; ceramic based on Pu-phosphate of monazite structure, PuPO4; ceramic based on zircon, (Zr,Pu)SiO4, and minor phase tetragonal zirconia, (Zr,Pu)O2; single crystal zircon, (Zr,Pu)SiO4; single crystal monazite, (Eu,Pu)PO4; ceramic based on Ti-pyrochlore, (Ca,Gd,Hf,Pu,U)2Ti2O7. No change of phase composition, matrix swelling, or cracking in cubic zirconia were observed after cumulative dose 2.77×1025 alpha decay/m3. The La-monazite remained crystalline at cumulative dose 1.19×1025 alpha decay/m3, although Pu-phosphate of monazite structure became nearly amorphous at relatively low dose 4.2×1024 alpha decay/m3. Zircon has lost crystalline structures under self-irradiation at dose (1.3-1.5)×1025 alpha decay/m3, however, amorphous zircon characterized with high chemical durability. The Ti-pyrochlore after cumulative dose (1.1-1.3)×1025 alpha decay/m3 became amorphous and lost chemical durability. Radiation damage caused crack formation in zircon single crystals but not in the matrix of polycrystalline zircon. Essential swelling and crack formation as a result of radiation damage were observed in ceramics based on Ti-pyrochlore and Pu-phosphate of monazite structure, but not so far in La-monazite doped with 238Pu.

Luminescent Properties in NIR Region of LaPO&lt;sub&gt;4&lt;/sub&gt;:Er/LaPO&lt;sub&gt;4&lt;/sub&gt; Core/Shell Nanoparticles

To get over the drawback of nanomaterials such as quenching by O-H group for EDWA application, LaPO 4 :Er/LaO 4 core/shell nanoparticles were synthesized in the mixture solvent system and their properties were investigated through various spectroscopic techniques. The prepared core/shell nanoparticles have featured by the single phase of monazite structure and about 7-8 nm of particle size without agglomeration. In the NIR emission spectrum, the LaPO 4 /LaPO 4 core/shell nanoparticles were largely increased in NIR-PL intensity due to encapsulation of the LaPO 4 shell and the extension of lifetime were observed. In addition, the co-doping Yb 3+ as a sensitizer of emission by Er 3+ in the core/shell nanoparticles was accomplished enlargement of emission intensity at 1539 nm due to the effective energy transfer from Yb 3+ to Er 3+ . On the other hand, the longest lifetime in the prepared core/shell nanoparticles was obtained at 1% activator concentration.

He diffusion systematics in minerals: Evidence from synthetic monazite and zircon structure phosphates

Helium diffusivity was measured in synthetic rare-earth-element orthophosphates with systematically varying properties to evaluate potential controls on He transport in minerals. In the zircon structure phosphates (in this study, the phosphates of Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as synthetic xenotime, YPO 4), He diffusion is strongly anisotropic. Transport apparently proceeds preferentially through channels aligned with the c-axis. The activation energy for diffusion is almost the same (122 ± 6 kJ/mol) in all members of this family, but there is a monotonic decrease in D o with atomic number from TbPO 4 (∼10 5 cm 2/s) to LuPO 4 (∼10 cm 2/s). The c-parallel channels become increasingly constricted in the same sequence, likely accounting for the systematically decreasing diffusivity. The He closure temperature ( r = 1 cm, d T/d t = 10 °C/Myr) increases with atomic number from 44 °C for TbPO 4 to 88 °C for LuPO 4. Diffusion of radiogenic helium from natural zircon and xenotime is much slower than these synthetic analogs predict, suggesting that coupled substitution of REE and P for Zr and Si and/or radiation damage profoundly modify the energetics of interstitial He diffusion. In particular, α-recoil may play a key role by damaging the continuity and integrity of the channels. Monazite structure phosphates (here La, Ce, Pr, Nd, Sm, and Gd phosphate) are far more He retentive than those of the zircon structure. Activation energies increase smoothly with atomic number from LaPO 4 (183 kJ/mol) to NdPO 4 (224 kJ/mol) then decrease again to GdPO 4 (198 kJ/mol). D o values mimic this pattern, spanning a range from ∼10 −1 cm 2/s (GdPO 4) to 10 4 cm 2/s (NdPO 4). Nevertheless, He closure temperatures increase monotonically with atomic number, from 300 °C in LaPO 4 to 410 °C in GdPO 4. No evidence was obtained bearing on diffusion anisotropy, but the monazite structure lacks through-going channels so it is not expected. Diffusion parameters for radiogenic helium in natural monazite are similar to those obtained on the synthetic analogs. Ionic porosity is not the primary control on He diffusion in the orthophosphates. Within a given structure and with limited elemental substitution, ionic porosity and He closure temperature are negatively correlated, as predicted. However, differences between crystal structures are far more important than ion packing density: at comparable ionic porosity the monazite structure phosphates have He closure temperatures ∼300 °C higher than the xenotime structure phosphates. Modifications to the structures by radiation damage likely play a similarly significant role in controlling He diffusion.

Plutonium and americium monazite materials: Solid state synthesis and X-ray diffraction study

High-temperature solid state syntheses of monazite powders containing plutonium (III), plutonium (IV) and americium (III) were performed. Resulting powders were characterized by X-ray diffraction. Pu 3+PO 4 was readily obtained as a single phase by heating a Pu 4+O 2–NH 4H 2PO 4 mixture under argon atmosphere. Traces of tetravalent plutonium phosphate Pu 4+P 2O 7 were detected when synthesized under air atmosphere. The incorporation of (Pu 4+, Ca 2+) in the monazite structure was investigated under air and argon atmosphere. We showed that Pu 4+ is fully reduced in Pu 3+ under argon atmosphere whereas, under air, the compound with the formula Pu 0.4 3 + Pu 0.3 4 + Ca 0.3 2 + PO 4 was obtained. Pure Am 3+PO 4 was also synthesized under argon atmosphere. X-ray patterns revealed a complete amorphisation of the monazite structure at a relatively low cumulative alpha dose for 241AmPO 4.

Electrical conduction properties of rare earth orthophosphates under reducing conditions

Electrical conduction properties of Sr-doped PrPO 4, NdPO 4 and SmPO 4 with the monazite structure were investigated at 500–925 °C under H 2/H 2O reducing conditions. From H/D isotope effects and p(H 2)-dependencies of the conductivities, it was found that the materials dominantly conducted protons under the wet reducing conditions although the materials slightly showed n-type electronic conduction at higher temperatures. Based on p(H 2O) and p(H 2)-dependencies of the conductivities, defect structures of the materials under the reducing conditions were discussed. As a result, it was considered that major positive defects were oxygen deficits at higher temperatures and protons at lower temperatures.

Microstructural and optical features of a Eu-monazite

The obtention of europium-phosphate nanoparticles by the precipitation method and its thermal evolution to become ceramics materials is presented. The monazite structure was obtained from the rabdophane phase after firing at 1000 °C during several hours. The powder characteristics made easy the pressing and sintering processes and it was possible to obtain high density bodies (relative density > 97%) at only 1200 °C. The fluorescence spectra and the lifetimes were investigated as a function of the heating temperature, as well, the microstructure and the residual glassy phase. It was found that higher sintering temperatures (>1500 °C) resulted in lower fluorescence emission than lower temperatures (maximum at 1200 °C) as consequence of the microstructure detrimental. The gamma irradiation up to the dose of 18 kGys did not produce any appreciable effect in the optical properties; however, the sintering temperature modified the optical absorption in the UV range.

The high-temperature heat capacity of LnPO 4 (Ln = La, Ce, Gd) by drop calorimetry

The high-temperature heat capacity of three lanthanide orthophosphates of monazite structure have been measured in the temperature range (450 to 1570) K. The data have been analysed in terms of a lattice term, represented by the 4f 0 and 4f 7 compounds LaPO 4 and GdPO 4, and an electronic term for CePO 4. The calculated excess heat capacity thus obtained is in reasonable agreement with that calculated from the crystal field energies.

Polyol-Mediated Synthesis and Photoluminescent Properties of Ce&lt;SUP&gt;3+&lt;/SUP&gt; and/or Tb&lt;SUP&gt;3+&lt;/SUP&gt;-Doped LaPO&lt;SUB&gt;4&lt;/SUB&gt; Nanoparticles

Rare-earth ion (Ce3+, Tb3+) doped LaPO4 nanoparticles were prepared by the polyol method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and lifetimes. The results of XRD indicate that the as-prepared nanoparticles are well-crystallized at 160 degrees C and assigned to the monoclinic monazite structure of the LaPO4 phase. The obtained LaPO4:Ce3+, Tb3+ nanoparticles are spherical with narrow size distribution and average size of 20 nm. The doped rare-earth ions show their characteristic emission in LaPO4 nanoparticles, i.e., Ce3+ 5d-4f and Tb3+ 5D4-7FJ (J = 6-3) transitions, respectively. The optimum doping concentration for Tb3+ in La(0.8-x)Ce0.2TbxPO4 nanoparticles is determined to be 15 mol% (x = 0.15). The luminescence decay curves of Ce3+ in LaPO4:Ce3+ and LaPO4:Ce3+, Tb3+ nanoparticles present a single-exponential behavior, and the lifetimes (tau) of Ce3+ decrease with increasing Tb3+ concentrations (at the constant Ce3+ concentration) in LaPO4:Ce3+, Tb3+ nanoparticles due to the energy transfer from Ce3+ to Tb3+. The energy-transfer efficiency from Ce3+ to Tb3+ was calculated, which depends on the doping concentrations of Tb3+ if the concentration of Ce3+ is fixed.

Environmentally benign novel green pigments: Pr1-xCaxPO4 (x = 0-0.4)

Rare earth based materials have recently attracted considerable attention as potential ecofriendly colourants for low temperature as well as high temperature applications. In the present study, we have synthesized a series of Ca-doped PrPO4 compounds with the general formula, Pr1-xCaxPO4 (x = 0-0.4 in steps of 0.1) and characterized the compounds by powder X-ray diffraction. All the compositions show a monoclinic monazite structure. The optical properties of the brilliantly coloured pigments [ L (brightness),a* (+ red- green),b* (+ yellow - blue) have been examined. These materials can find application as potential green colourants.

Preparation and luminescence properties of LaPO 4:Er,Yb nanoparticles

For possible applications as optical amplification materials in telecommunications, LaPO 4:Er,Yb nanoparticles were synthesized in a solution system and their properties were investigated by various spectroscopic techniques. The prepared nanoparticles are single-phased and present the monazite structure, the particle size being about 5 nm with a narrow size distribution. Also, it was confirmed by EA and FT-IR analyses that the surface of nanoparticles is coated with the solvent molecules used in the synthesis reaction, which possibly prevents them from agglomerating. In the NIR region, the emission of the LaPO 4:Er particles is very weak due to an efficient quenching of the 4I 13/2→ 4I 15/2 emission by the hydroxyl groups adsorbed on the surface of the nanoparticles. On the other hand, the co-doping of Yb 3+ as a sensitizer in the nanoparticles resulted in the increase of the emission intensity at 1539 nm due to the effective energy transfer from Yb 3+ to Er 3+. In addition, the synthesized nanoparticles have exhibited good dispersibility into a polymer matrix and effective luminescence in the NIR region.

Synthesis and characterization of uranium (IV) phosphate-hydrogenphosphate hydrate and cerium (IV) phosphate-hydrogenphosphate hydrate

A new uranium (IV) phosphate of proposed formula U 2(PO 4) 2HPO 4·H 2O, i.e. uranium phosphate-hydrogenphosphate hydrate (UPHPH), was synthesized in autoclave and/or in polytetrafluoroethylene closed containers at 150 °C by three ways: from uranium (IV) hydrochloric solution and phosphoric acid, from uranium dioxide and phosphoric acid and by transformation of the uranium hydrogenphosphate hydrate U(HPO 4) 2· nH 2O. The new product appears similar to the previously published thorium phosphate-hydrogenphosphate hydrate Th 2(PO 4) 2HPO 4·H 2O (TPHPH). From preliminary studies, it was found that UPHPH crystallizes in monoclinic system ( a = 2.1148 ( 7 ) nm , b = 0.6611 ( 2 ) nm , c = 0.6990 ( 3 ) nm , β = 91.67 ( 3 ) ° and V = 0.9768 ( 10 ) nm 3 ). Heated under inert atmosphere, this compound is decomposed above 400 °C into uranium phosphate-triphosphate U 2(PO 4)P 3O 10, uranium diphosphate α -UP 2O 7 and diuranium oxide phosphate U 2O(PO 4) 2. Crystallized cerium (IV) phosphate-hydrogenphosphate hydrate Ce 2(PO 4) 2HPO 4·H 2O (CePHPH) was also synthesized from (NH 4) 2Ce(NO 3) 6 and phosphoric acid solutions by the same method (monoclinic system: a = 2.1045 ( 5 ) nm , b = 0.6561 ( 2 ) nm , c = 0.6949 ( 2 ) nm , β = 91.98 ( 1 ) ° and V = 0.9588 ( 9 ) nm 3 ). When heating above 600 °C, cerium (IV) is reduced into Ce (III) and forms a mixture of CePO 4 (monazite structure) and CeP 3O 9.